Broad-area illumination systems are used in a wide range of applications, e.g., backlighting of signs and graphic panels, as well as backlighting of decorative, non-information-carrying panels, such as translucent stone, luminous ceilings, luminous walls, and the like. Such applications may involve illumination of hundreds or thousands of square feet and thus require a large number of individual light sources. Typically, lighting solutions for these applications have a number of competing requirements, including uniform illumination (both intensity and color), thin system profile, the ability to customize both the size and shape of the illuminated structure, high efficiency, and low cost. Systems designed for outdoor, wet, or other challenging environments have additional requirements related to protection of the lighting system.
These needs are often at odds with each other. For example, variations in light and/or color intensity may be mitigated by having a deep (i.e., thick) mixing chamber to homogenize the light among the illumination sources within the system, but this makes the overall system significantly thicker. Thin systems typically require a larger number of light sources in order to reduce the mixing chamber depth, which increases the cost.
Uniform illumination across custom shapes and sizes is particularly difficult to achieve for a number of reasons. Incandescent and fluorescent light sources come in fixed sizes, limiting the granularity of the illumination sources—for example, fluorescent tubes come in fixed lengths and cannot be cut to length. Lighting systems based on light-emitting diodes (LEDs) typically are mounted on a circuit board having an electrical topology not amenable to being cut to length. For example, for large-area lighting applications, LEDs may be electrically laid out in groups of series-connected strings, e.g., on a square or rectangular tile, where each string contains multiple series-connected LEDs all operating at the same current. While in some topologies, one or more strings may be removed to permit dimensional customization, parts of a string typically cannot be removed without opening the circuit and causing de-energization of that string. The physical layout of the string therefore may limit the level of achievable granularity. Once installed in a lighting system, such fixed size illumination sources may visually result in regions having undesirably different light intensity levels or colors. A second issue with removing LED strings is that such systems are typically driven by a constant-current driver, so when one or more strings are removed, the current from the driver is divided among fewer strings, resulting in a local brightness increase. The lack of granularity in the sizing of the illumination sources and/or possible current variations between LEDs or groups of LEDs may result in visually distinguishable variations in light intensity level and/or color, for example correlated color temperature (CCT). From an application perspective, this is undesirable because the illumination level is desirably uniform over the entire illuminated area.
Another electrical topology that may be utilized for LED-based illumination sources is the connection of all of the LEDs in parallel. This topology may permit removal of individual LEDs and thus may achieve relatively finer granularity, in some cases on the order of the LED spacing. However, such systems are prone to “current hogging,” in which the current preferentially flows through the LED(s) with the lowest forward voltage. This can result in increased heating of such LEDs, which further reduces the forward voltage, thus increasing the current—this process can continue until those LEDs fail. In some cases, this process may occur over and over, for example cascading from one LED to the LED having the next lowest forward voltage in the system. In some cases, this effect may be mitigated by carefully matching the forward voltage of all of the LEDs, but this typically adds significant expense. Another approach is to incorporate a ballast resistor or other current-limiting device with each LED; however, this may increase cost and significantly reduce efficiency because of the power loss in the ballast resistor. A further efficiency disadvantage of this electrical topology is that it typically is driven at about the forward voltage of one LED. Low-voltage systems typically have increased power losses in the lines (wires) as well as lower driver efficiency.
A third electrical topology, using a constant-voltage supply in combination with an array, for example a parallel array, of small, low-cost LEDs configured in strings of series-connected LEDs, where each string also includes a current-regulating element, addresses a number of the deficiencies of the systems described above. Exemplary electrical and physical schematics of this approach are shown in FIGS. 1A and 1B respectively. This approach is described in detail in U.S. patent application Ser. No. 13/799,807, filed on Mar. 13, 2013, (the '807 application) and U.S. patent application Ser. No. 13/970,027, filed on Aug. 19, 2013 (the '027 application), the entirety of each of which is incorporated herein by reference. Such as lighting system may include power conductors 120 and 121, which supply power to strings 150. Each string 150 includes light-emitting elements (LEEs) 140 electrically connected in series by conductive elements 160 and energized by power from power conductors 120 and 121. Each string 150 also includes a current control element 145, which regulates the current in each string. In various embodiments strings 150 may be electrically connected in parallel with each other. One or more strings may be removed from the system without changing the brightness of the LEEs in the other strings. Strings may be straight, as shown in FIG. 1A, or folded, as shown in FIG. 1B (a folded string has multiple segments (in FIG. 1B they are parallel) between power conductors 120 and 121. (Here folded refers to the physical layout of the strings, rather than any particular geometric configuration of the system itself) In this example, power conductors 120, 121, conductive elements 160, control elements 145 and LEEs 140 are mounted on substrate or circuit board 165. The system granularity is determined at the base level by the LEE pitch 125. For a straight string, the size increment is on the order of pitch 125. For a folded string, the size increment is on the order of an integer multiple of pitch 125. The relatively small pitch permits a relatively thin system, for example including the light source and an overlying diffuser or optic, for example on the order of about 1.5 or 2 times the pitch distance.
The system described in relation to FIGS. 1A and 1B permits customization in one direction. For example, this may be used to produce a linear illuminated region having the sheet width, with the length customized by cutting off one or more strings. However, many applications require area lighting, such as shown in FIG. 1C, in which the desired illuminated area is not an integer multiple of the sheet width and/or length. In this case, fixed-size pieces or sheets of lighting system 110 may be tiled together to cover a desired portion 172 of a total area 170. A cut-to-length system may be used to tile regions 174 and 176, the dimensions of which are smaller in one direction than the tiles used in region 172. However, region 178, which requires cutting in two directions (in this case in substantially orthogonal directions), typically cannot be illuminated via this approach. That is, both dimensions of region 178 are smaller than those of available light-system sheets 110, which will not function properly if cut along both dimensions to the desired size. Similar challenges arise for non-rectangular shapes, such as those shown in FIGS. 1D and 1E. In these cases, square or rectangular tiles may be used to fill up most of the area, but undesirably non-illuminated regions remain. Furthermore, area lighting systems may be required to accommodate various penetrations or holes in the lighting plane, or regions without one or more illumination sources, for example for fire suppression systems such as sprinkler heads or the like, smoke or fire sensors, cameras, heating, ventilation and air conditioning ducts, supports or stand-offs for overlying material such as diffusers, optics, fabric or the like.
In view of the foregoing, a need exists for systems and procedures for thin, low-cost lighting systems enabling uniform illumination of arbitrarily sized areas.